In the past funding period, we have identified strong associations between circulating levels of branched- chain amino acids (BCAA-Leu, Val, He) and chronic metabolic diseases in multiple human cohorts. In each case, principal component analysis identified a correlating group of metabolites comprised of all three BCAA, aromatic amino acids (Phe, Tyr), and C3 and C5 acylcarnitines, with disease associations much stronger than for any lipid-related principal component. Feeding studies involving BCAA supplementation of high fat diets demonstrated a contribution of BCAA to development of insulin resistance independent of body weight. However, the increases in BCAA, aromatic amino acids, and related metabolites that we observe in the blood of humans with metabolic diseases is not necessarily driven only by protein consumption, and could also be influenced by rates of amino acid catabolism and protein turnover, or changes in hormones, such as the substantial decrease in IGF-1 levels that we observe in obese humans. Herein, and in close collaboration with the other projects and cores, we will test an evolving model for BCAA-mediated impairment of insulin action involving decreased disposal of BCAA in adipose tissue, and consequent accumulation of BCAA metabolites in skeletal muscle. The role of decreased lGF-1 levels in control of amino acid homeostasis will also be studied. The overarching goal of Project 1 is to fully understand the metabolic and molecular changes that lead to perturbed BCAA homeostasis and loss of insulin sensitivity in animal models, thereby leading to better understanding of possible cause/effect relationships between BCAA and metabolic disease in human subjects. Specific aims are: 1. To study the time course of changes in pathways of BCAA metabolism during development of insulin resistance in Zucker-obese rats; 2. To expand upon our prior studies of dietary supplementation of BCAA in normal rats to include Leu alone and aromatic amino acids; 3. To investigate the impact of three maneuvers designed to reverse defects in BCAA metabolism and insulin resistance in Zucker-obese rats; 4. To investigate the effects of HF or HF + BCAA feeding on insulin sensitivity, BCAA levels, and BCAA metabolism in mice with reduced circulating IGF-1